1. Field of the Invention
The present invention relates to a Global Positioning System (GPS). In particular, the present invention relates to an apparatus and method for improving GPS reception sensitivity in a mobile terminal with a mobile-based GPS positioning function.
2. Description of the Related Art
Modern society's development is a driving force for drastic growth in personal portable communications and provisioning of various additional services. In some countries, mobile terminals are equipped with a Global Positioning System (GPS) and positioning information-related services are provided to the mobile terminals. Many GPS satellites broadcast their ephemerides and system time, while circling the earth in predetermined orbits, so that a GPS receiver can determine its location. A GPS receiver in a mobile terminal determines accurate time and its location by calculating relative time of arrivals of GPS signals simultaneously transmitted by four or more GPS satellites.
GPS-based positioning is a process of measuring time delays of signals from the satellites and calculating the current position by using the time delays and the distances to the orbits of the satellites. When the distance to a first satellite is known, the GPS receiver's position is somewhere on the surface of a sphere with a radius equal to the distance, centered on the first satellite. When the distance to a second satellite is known, the GPS receiver's position is somewhere on the circle formed by the intersection of the sphere centered on the first satellite and the sphere centered on the second satellite. Using the distance to a third satellite, the third satellite results in a sphere that intersects this circle at two points. The GPS receiver's position is one of these two points. The distance to a satellite measured by the GPS receiver is referred to as a pseudo-range because it is a time-error biased distance measurement having a time error-caused distance error added to a true distance calculated by using the time it takes for the signal to travel from the satellite to the GPS receiver. Therefore, the GPS receiver eliminates the time error by using the distance to a fourth satellite.
GPS positioning often takes several minutes. This time consumption is not acceptable especially to a small-size GPS receiver provided in a mobile terminal with a limited battery life. Thus, some GPS receivers receive basic information necessary for satellite search from a nearby Assisted GPS (AGPS) server, such as a Position Determination Entity (PDE) server. The basic information is provided to mobile terminals by a base station in a network that serves the mobile terminals. For example, a synchronous Code Division Multiple Access (CDMA) radio network supports IS-801 as a message protocol between a mobile terminal and the PDE server, for a location-based service.
GPS-based positioning services are as follows.
1. Mobile-Assisted Positioning
The mobile-assisted positioning is the most popular positioning service defined by IS-801. The PDE server provides the mobile terminal with Acquisition Assistance (AA) data and Sensitivity Assistance (SA) data as information needed for positioning. The mobile terminal measures the pseudo-ranges to satellites by the shortest GPS operation based on the AA data and the SA data. The PDE server receives the pseudo-range measurements from the mobile terminal over a radio network and calculates the position solution of the mobile terminal.
The mobile-assisted positioning technology is useful for the case where the network detects the position of the mobile terminal, as in the E911 service. However, because one position solution is obtained at one time in the positioning service, it is not applicable to the case where continuous mobile position solutions are needed, like navigation.
2. Mobile-Based Positioning
Compared to mobile-assisted positioning, in the mobile-based positioning technology, the mobile terminal receives the ephemerides of satellites, almanac data, and its coarse position information from the PDE server, generates AA data, and calculates the position solution based on the pseudo-ranges to the satellites measured by using the AA data. Despite the shortcoming of GPS operation complexity, the mobile-based positioning is suitable for the case where the position solution must be continuously calculated.
The SA data refers to data that the mobile terminal selectively requests to the PDE server and receives from the PDE server in the mobile-assisted positioning service. The SA data comprises about 10 seconds of GPS navigation information bits. The use of the information bits increases a coherent integration time, thereby improving GPS reception sensitivity.
If the position solution is continuously calculated in the mobile-based positioning service, the GPS operation continues for a long time in most cases. The 10 seconds of SA data transmitted from the PDE server to the mobile terminal does not provide sufficient information bits, and thus is not available in the mobile-based positioning service. However, the absence of the navigation information bits restricts the increase of coherent integration time. Thus, the mobile terminal fails to acquire or misses satellite signals. In other words, the GPS reception sensitivity is decreased.
FIG. 1 is a block diagram illustrating a conventional mobile-based GPS receiver. Referring to FIG. 1, a GPS satellite 102 transmits a GPS signal by two carriers for standard positioning and precise positioning, L1 and L2. The GPS signal reaches a mobile terminal 142 and a PDE server 104. The PDE server 104 acquires ephemeris data comprising satellite orbit information such as the current position and speed of the GPS satellite 102, almanac data, and a coarse position solution by demodulating the GPS signal, and generates basic information based on those data. The basic information is provided to a base station 108 that covers a cell in which the mobile terminal 142 is located through a mobile switching center (MSC) 106 in a CDMA network 144. The base station 108 in turn transmits the basic information to the mobile terminal 142 by a CDMA signal.
In the mobile terminal 142, an antenna 110 receives radio frequency (RF) signals from the GPS satellite 102 and the CDMA base station 108. A duplexer 112 transfers the RF signals to a CDMA RF receiver 114 and a GPS RF receiver 118. The CDMA RF receiver 114 extracts the CDMA signal from the RF signals. A CDMA baseband processor 116 acquires the basic information transmitted by the PDE server 104 and system time information transmitted by the CDMA network 144 by demodulating and decoding the CDMA signal. A Temperature-Compensated Crystal Oscillator (TCXO) 120 provides a reference clock signal to the CDMA RF receiver 114, the GPS RF receiver 118, and a GPS baseband processor 122, in synchronization to the acquired system time.
The GPS RF receiver 118 extracts the GPS signal from the RF signals. The GPS signal is provided to a mixer 130 in the GPS baseband processor 122. A carrier Numerically Controlled Oscillator (NCO) 124 in the GPS baseband processor 122 generates a carrier frequency signal for the GPS signal in synchronization to the reference clock signal received from the TCXO 120. Similarly, a code NCO 126 generates a code frequency signal in synchronization to the reference clock signal. A code generator 128 generates the Pseudo-Random Noise (PRN) code of the GPS satellite 102 according to the code frequency signal.
The mixer 130 generates a baseband signal by mixing the GPS signal with the carrier frequency signal. A correlator 132 receives the baseband signal and the PRN code. The GPS baseband processor 122 generates AA data by using the basic information, creates a replica of the true GPS signal from the GPS satellite 102 by using the AA data, and correlates the replica signal with the true GPS signal through the correlator 132. That is, the correlator 132 correlates the GPS signal received from the mixer 130 with the replica signal by using the PRN code. The correlator 132 integrates the correlation result for a predetermined coherent integration time, for example, 1 millisecond and then outputs correlation samples.
A memory 134 stores the correlation samples. A peak detector 136 detects samples having peak energy equal to or greater than a predetermined threshold among the stored samples. A pseudo-range measurer 138 calculates the pseudo-range to the GPS satellite 102 using the detected samples. A position solution calculator 140 calculates the position solution of the mobile terminal 142 using pseudo-range measurements to at least four GPS satellites and ephemeris data provided in the basic information.
As stated before, SA data is data that the PDE server 104 transmits to the mobile terminal 142 in order to increase GPS reception sensitivity in mobile-assisted positioning. The SA data contains information bits of GPS navigation data that the mobile terminal 142 is to receive. The information bits of the navigation data are known by the SA data, thereby increasing the coherent integration time. Without the SA data, the mobile terminal 142 can perform an integration within 20 milliseconds which is the duration of one data unit. With the SA data, the coherent integration time can be increased to 60 to 100 milliseconds. In the case of a long time coherent integration, the use of the SA data increases the GPS reception sensitivity by 2 to 3 dB, relative to non-use of the SA data.
Unlike mobile-assisted positioning, in the mobile-based positioning, the SA data is not provided from the PDE server 104 to the mobile terminal 142. Even though the mobile terminal 142 receives the SA data by requesting it from the PDE server 104, the SA data is effective for only about 10 seconds and thus is not suitable for the mobile-based positioning which requires continuous computation of the position solution of the mobile terminal 142. In the conventional mobile-based positioning, therefore, due to the absence of SA data, a coherent integration does not last 20 milliseconds or longer. Consequently, a decrease by 2 to 3-dB in sensitivity is observed, compared to mobile-assisted positioning. The sensitivity decrease is a big problem for mobile-based positioning in GPS navigation applications for mobile terminals.